In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Overlay materials are used in dentistry in order to obtain natural-looking dental restorations. Glass-ceramics, also known as porcelains in the art, and certain glasses, are desirable for this purpose since they can be colored to closely resemble the teeth they must replace, resist degradation inside the oral cavity, and remain biocompatible even after years of continuous contact with mammalian tissue. The term “dental ceramics,” is usually used to refer to both glass and glass-ceramic overlay materials, as well as glass-ceramic and crystalline ceramic materials used for dental restoration cores and frameworks. The term “porcelain” is often used interchangeably with the term “glass ceramic” to designate dental ceramics comprising both glass and crystalline phases. Dental restorations or prostheses comprising dental ceramics may be classified as either metal-ceramic or as all-ceramic restorations.
Today, there is an increasing trend in dentistry toward the use of ceramic cores or frameworks in lieu of metal alloy frameworks to provide an all-ceramic dental restoration, which is associated with better aesthetics and biocompatibility.
Yttria stabilized tetragonal zirconia polycrystalline (YTZP) has emerged as a high-strength core or framework material for aesthetic dental restorations (e.g., crowns, partial crowns and bridges). In particular, shaded YTZP frameworks have become favored for their aesthetic appeal in the fabrication of all ceramic dental restorations. Consequently, a large number of CAD/CAM systems capable of fabricating YTZP zirconia frameworks have entered the market. Because of high costs of acquiring and operating CAD/CAM systems an alternative trend has emerged. Namely, analog mechanical systems capable of fabricating oversized shapes from partially sintered zirconia using pantograph-like devices, which is described, for example, in U.S. Pat. No. 5,788,498. The accuracy of these relatively low-tech devices is inferior to the state of the art CAD/CAM systems.
However, even the best CAD/CAM system is not able to match pressable ceramics in terms of accuracy of margin adaptation. Quality and accuracy of margins, i.e., margin fit, delivered by alternative pantograph-based systems are not even remotely comparable to benchmarks achievable by heat pressing methods.
Another issue that complicates the fabrication of an aesthetically superior restoration with a zirconia core is the fact that most of the systems require a liner as an intermediate layer between zirconia cores or frameworks and an overlay ceramic. The liner imparts or modifies the color of the framework which is often stark white, and also improves the bonding between the overlay and zirconia. However this liner, which thickness often approaches 0.1 mm, reduces the perceived translucency of the zirconia core and takes invaluable clinical space required for layering.
U.S. Patent Application Publication 2004/0232576 teaches making pressable pellets or ingots from dental porcelain powders comprising alumo-boro-silicate glass and pressing these pellets onto various cores/frameworks including zirconia cores using conventional heat pressing techniques and a commercial dental pressing furnace. These pressable materials have a coefficient of thermal expansion (CTE) in the range of 7-13×10−6/° C. The maturing temperatures and therefore pressing temperatures of dental porcelain compositions disclosed in this publication are less than about 850° and are not optimal for pressing onto YTZP frameworks. It is also difficult to mass produce pellets or ingots from powders comprising low-fusing glasses (with maturing temperature <850° C. as taught in US 2004/0232576) due to difficulties in clean binder removal without graying.
U.S. Patent Application Publication 2006/0099552 teaches heat pressing of tooth colored glass onto YTZP frameworks wherein pressing temperatures are in the range of 750°-1000° C. Glasses disclosed in both patent applications are known to be inherently weak and thus limit the strength of the resulting overlay.
On the other hand, margin porcelains are known to have a relatively high flexural strength, in the range of 120-150 MPa, compared to conventional overlay porcelains. It is also a well known concern among clinicians that an overlay is a weak link in a zirconia-core dental restoration which can limit the structural integrity and life span of the restoration. Therefore, if a pressable overlay material is also used for pressing margins it is extremely desirable that it has strength comparable to or exceeding that of margin porcelains.
It should be noted that “heat pressing,” also known as (low pressure) injection molding in the art and commonly referred to as simply “pressing,” is different from “hot pressing.” Conventional heat pressing is widely used in commercial dental labs in conjunction with mass-produced equipment and requires relatively low pressures of 0.2-0.7 MPa. Hot pressing, despite having a nearly identical name, requires more sophisticated and often experimental or unique equipment and 20-100 times higher pressures, i.e., in the range of 10-100 MPa. Heat pressing is well described in U.S. Pat. Nos. 6,484,791 and 6,302,186. Hot pressing as it relates to fabrication of dental restorations is described in U.S. Pat. Nos. 5,849,068, 5,916,498 and 6,126,732. These patents teach hot pressing of mixtures of oxide ceramics particles and from 1 wt. % to 50 wt. % of glass particles which were added to impart some pressability, at least at high pressures. Pressing takes place at temperatures from 800° C.-1300° C. and pressures from 10 MPa to 40 MPa. The materials disclosed in the above patents are not pressable using conventional pressing technique because they comprise 50-99 wt % of crystalline oxide ceramics particles which inhibit flow under low pressures, but are used to provide a relatively high strength exceeding 300 MPa.
The aforementioned U.S. Pat. Nos. 5,788,498 and 5,833,464 teach hot-pressing a dental ceramic onto prefabricated ceramic reinforcement members, or a pin/post, respectively. Both patents claim zirconia (zirconium oxide) as the preferred material for reinforcement member or pin/post. It should be noted that the reinforcing members and pin/post construction described in these patents are distinct from the core and framework construction of the present invention.
Thus, a need exists in the art for pressable overlay materials for various uses, such as for pre-shaded YTZP zirconia frameworks that do not require use of a liner and have sufficient strength to fabricate 360° overlays, or at least buccal/labial porcelain margins yielding the required variety of shades consistently and without compromise in the structural integrity of the resulting dental restoration. A need also exists for the ability to press an overlay material directly onto YTZP cores or frameworks resulting in net shape, full contour dental restorations with fully formed margins, occlusal and mesio-distal surfaces wherein shade, translucency, CTE and strength of the pressed overlay are optimized, thereby eliminating a porcelain layering step to form such margins and/or surfaces from separate materials. Moreover, a need exists to carry out pressing in the temperature range that is most favorable for YTZP cores or frameworks, thereby promoting retention of their high strength.